Rotary drive systems typically include a screw threadably mated with a nut, wherein relative rotation between the screw and nut is accompanied by axial displacement between the screw and nut. This axial displacement is used to drive a linear stroke of an actuating mechanism. In such rotary drive systems, it is often necessary to provide a stop module to mechanically limit the linear stroke extent by accurately limiting the number of revolutions the system can rotate in either rotational direction or in both rotational directions.
FIG. 1 illustrates a rotational drive system 1 comprising an input shaft 2 rotatable about an axis 3 and arranged to drive rotation of output shaft 4 about axis 3. Output shaft 4 may be used, for example, to supply rotational power for driving the stroke of an actuator rod (no shown). Rotational drive system 1 also comprises a currently known stop module 5 of the prior art. Stop module 5 includes a travelling nut 6 on a screw shaft 7 mounted to rotate with input shaft 2, wherein the nut 6 moves axially with each revolution of input shaft 2 and screw shaft 7. When it reaches a travel limit end, nut 6 compresses a stack of alternating rotor/stator brake plates 8A, 8B and several Belleville springs 9 to arrest the rotating motion of input shaft 2 (i.e. “lock up”). Nut 6 includes an Acme screw thread 6A to allow stop module 5 to overcome static friction and unlock when input shaft 2 is rotated in the opposite direction after a lock-up event. Brake plates 8A, 8B introduce undesired power losses into the system in the form of viscous drag, particularly at low temperatures. Brake plates 8A, 8B are also subject to wear over time, which can alter the stroke resulting length and lead to over-travel beyond the intended limit.
There is a need for an improved stop module that significantly reduces the viscous drag imposed on the rotational drive system, particularly at low operating temperatures, increases the overall efficiency of the system, and eliminates loss of accuracy in the predetermined travel limits due to wear.